A supplementary cover is a cover that is not necessary for the safe use or proper functioning on an electronic device but is applied over at least a portion of an existing cover for some additional purpose e.g. to protect some or all of the electronic device from damage.
A supplementary cover may be distinguished from a replacement cover. A replacement cover is a cover for the safe use or proper functioning of an electronic device and replaces an existing cover e.g. to change the aesthetic appearance of the electronic device.